1. Field of the Invention
The invention relates to a single stage power factor correction (PFC) converter, and more particularly to a single stage PFC converter with constant voltage and constant current output for power supply, battery charger and LED lighting driver, etc.
2. Description of the Related Art
FIG. 1 shows a prior art of an offline power converter. The offline power converter comprises a transformer 10, a power transistor 20, a resistor 30, an input bridge-rectifier 35, a diode 41, a high input electrolytic capacitor 43, a capacitor 45, a switching controller 100, resistors 51 and 52, a diode 60, and a capacitor 65. The transformer 10 includes a primary winding NP, an auxiliary winding NA, and a secondary winding NS. The high input electrolytic capacitor 43 is used for the energy storage. Waveforms of an input line voltage VAC, an input line current IAC, and an input voltage VIN in FIG. 1 are shown in FIG. 2. The input voltage VIN is the voltage on the high input electrolytic capacitor 43. The high capacitance input capacitor 43 will cause distortion of the input line current IAC and generate poor power factor (PF). Therefore, the capacitance of the high input electrolytic capacitor 43 must be reduced to improve the power factor. However, having no input capacitor with high capacitance will produce a low input voltage VIN. The low voltage of the input voltage VIN will cause feedback open loop for the offline power converter. An output voltage VO of the offline power converter can be expressed as,
                              V          O                =                  N          ×                      V            IN                    ×                                    T              ON                                      T              -                              T                ON                                                                        (        1        )            where N represents a turn ratio of the transformer 10 (N=NS/NP; Np is the primary winding, and NS is the secondary winding); VIN represents the input voltage of the transformer 10 (also the voltage on the high input electrolytic capacitor 43); TON represents on-time of a switching signal SW which controls the power transistor 20; T represents a switching period of the power transistor 20.
In order to achieve a stable feedback loop and prevent transformer saturation, the maximum duty cycle “TON/T” is limited, such as <80% in general. If the input voltage VIN is too low, the maximum on-time TON of the switching signal SW will be unable to maintain the regulated output voltage VO (shown in equation (1)) and cause the feedback open loop. When the feedback loop is significantly on/off (close-loop and open-loop) in response to the change of the input line voltage VAC, an overshoot and/or undershoot signal can be easily generated at the output of the power converter. Besides, the input capacitor 43 is an electrolytic capacitor that is bulky and has low reliability. The object of this invention is to improve the power factor of the power converter without the need of extra power factor correction (PFC) power stage. Another object of this invention includes eliminating the need of the input electrolytic capacitor for improving the reliability of the power converter and reducing the size and the cost of the power converter.